1. Field of the Invention
The invention relates to a method of producing homogeneously doped n-type Si monocrystals and adjusting dopant concentration within such crystals and somewhat more particularly to a method of producing such crystals and adjusting dopant concentrations therein by controllably irradiating p- or n-type Si monocrystals having a random dopant concentration in radial and axial directions thereof with thermal neutrons in accordance with the reaction: EQU Si.sup.30 (n,.gamma.)Si.sup. 31 .fwdarw..sup..beta. P.sup.31. (I)
2. prior Art
Si crystal bodies, such as rods, are generally doped after the precipitation or deposition of solid Si onto a heated mandrel or rod-shaped carrier member with the aid of thermal and/or pyrolytic decomposition of gaseous Si compounds. In such a process, dopants are intermixed with a gaseous Si compound and decompose at the carrier member so as to be dispersed within the forming Si body. The Si rods or bodies so produced are polycrystalline and must be converted into a monocrystalline state by subsequent zone melting processes. During such zone melting, the concentration of the dopant within the Si rod often changes in an uncontrolled manner and considerably higher dopant concentrations must be provided in the polycrystalline rods in order to attain a desired dopant concentration in the final crystalline rods, especially in instances where a plurality of zone melting processes are utilized. The silicon crystal rods of the n-type produced in this manner exhibit radial and axial fluctuations in specific resistance at micro areas of the crystal. Such fluctuations can be traced back to the fluctuation of dopant concentration therein which is primarily imparted during the crystal growth process. Fluctuations in specific resistance cause considerable drawbacks, particularly in the production of highly blocking semiconductor components.
M. Tanenbaum et al, "Preparation of Uniform Resistivity n-Type Silicon by Nuclear Transmutation", Journal of Electrochemical Society, Vol. 108 No. 2, Pages 171-176 (February 1961), suggests that Si crystals having n-type conductivity may be produced by radiation of thermal neutrons on pure Si crystals. In this process, the natural isotope Si.sup.30, which is present in pure Si crystals, is transmuted into the unstable isotope Si.sup.31 by the capture of a thermal neutron and emission of .gamma. radiation. The unstable Si.sup.31 isotope decays by .beta..sup.- emission with a 2.62 Hr. half-life into the stable P.sup.31 isotope.
From the foregoing, a so-called radiogeneous doping of silicon may be achieved in accordance with the nuclear reaction: EQU Si.sup.30 (n,.gamma.)Si.sup. 31 .fwdarw..sup..beta. P.sup.31 (I)
so that by presuming that the entire amount of Si.sup.31 is completely decayed and that the transmutation loss of Si.sup.30 is negligibly small, the following simple computation is valid: EQU C.sub.P = 1.7.times. 10.sup.-.sup.4 .times. .PHI. .times. t (II)
wherein C.sub.P is the phosphorous concentration in atoms/cm.sup.3 ; .PHI. is the thermal neutron flux in neutrons/cm.sup.2 .times. sec; and t is the radiation time in seconds.